1. Field of the Invention
The invention is generally directed to surface covering products such as floor tile and floor covering sheets. More particularly, the present invention relates to a wearlayer surface covering in which the wearlayer is coated onto a substrate and initially is cured with low peak irradiance UV energy and thereafter further cured with high peak irradiance to form a low gloss wearlayer surface coating.
2. Description of the Related Art
Cured coatings such as floor wearlayer surfaces function to provide scratch resistance and durability to flooring. The durability encompasses both wear and tear properties, and wearability includes scratch resistance and gouge resistance. Further, the property of scratch resistance includes the ability to withstand both fine and deep scratches from soils and grit particles resulting from foot traffic. Additionally, the property of gouge resistance includes the ability of the coating and flooring to withstand damage from foot traffic in the form of heel damage or dent damage from objects striking the flooring.
For many of the decorative patterns in the flooring industry, the desirable coating is a wearlayer surface that has a glossy appearance. In other cases, low gloss is desirable for natural decorative patterns such as wood, slate or brick. A low gloss or matte coating is more preferable to give a xe2x80x9cnatural or realxe2x80x9d appearance of the decorative layer. Accordingly, the gloss level of the coating must be specific for individual decorative patterns and therefore, it becomes a processing requirement to be able to control the gloss level of the coating for specific decorative patterns.
Furthermore, manufacturing differences in producing floor covering products may cause visible differences from batch to batch. Thus, in order to maintain a consistent gloss level throughout subsequent batches of floor covering products, gloss-level control must be addressed in the manufacturing process.
A prior art approach to obtaining low gloss coatings typically requires multiple ultraviolet (UV) medium pressure-mercury vapor lamps to cure a gloss-forming coating on the flooring material. The UV lamps typically have a long wavelength and a short wavelength output. To match the UV output of these sources, photoinitiators can be used to absorb UV light above 300 nm and below 300 nm. Microwave UV source lamps that exhibit characteristic spectral distributions above 300 nm, however, are more costly than standard medium pressure-mercury vapor lamps. Additionally, some of these photoinitiators (commonly referred to as phosphine oxides) are costly, and in many instances, have the problem of leaving an undesirable yellow color to cured final product.
U.S. Pat. No. 4,313,969 discloses a method to prepare low gloss or gloss controlled radiation coating by utilizing a two stage process. The first stage of the method involves UV cure of the resin material utilizing long wavelength UV spectral output where no substantial spectral distribution exists below 300 nm. The second stage of the method involves the curing the resin under a source comprised of short wave radiation where substantial spectral intensity is below 300 nm.
U.S. Pat. No. 5,585,415 teaches a method of preparing a low gloss coating by utilizing a three stage process including the steps of: (1) passing the coated substrate under ionizing radiation from an electron beam; (2) passing the coated substrate under UV lamps under an oxygen-containing atmosphere; and (3) passing the substrate under UV lamps in an inert atmosphere. The ""415 patent discloses the use of a pigmented resin material that contains a phosphine oxide photoinitiator to provide a thorough curing.
U.S. Pat. No. 4,048,036 teaches a method to prepare low gloss coatings derived from a resin composition containing a flatting agent and air cure promoter, disclosed as benzophenone, and a second photoinitiator. The resin material is exposed to UV lamps in a high oxygen atmosphere to provide a wet surface and a partially cured, or xe2x80x9cgel,xe2x80x9d state followed by exposure to UV lamps in a low-oxygen atmosphere to fully cure the coating.
The present invention is based on a composition and method of making a surface covering product, and preferably a floor covering product, in which the composition is selectively curable to either a high or low gloss wearlayer. The gloss is controlled by the use of low peak irradiance UV energy under an atmosphere of ambient air, followed by exposure to high peak irradiance UV light, preferably under an inert atmosphere comprised of nitrogen. The inventive composition contains a conventional flatting agent to lower gloss and/or hard particulates to improve abrasion resistance of the final surface wearlayer.
In the preferred embodiment, gloss levels for the same composition can be controlled by varying UV peak irradiance. The level of peak irradiance for a given coating composition enables specific process parameters to control gloss level. The peak irradiance of the initial UV exposure can be changed, for example by varying the distance between the source and substrate, or alternately, by changing input power for the UV source.
In the preferred process of making the surface covering, the gloss controlled wearlayer composition is formed from an acrylated aromatic polyester in the presence of multifunctional acrylates and a flatting agent and/or hard particulate materials. Partial polymerization is catalyzed by low peak irradiance UV radiation to gel the coating to form a low gloss wearlayer surface. Further polymerization is catalyzed by either high peak irradiance UV radiation, or alternately, by low energy electron beam radiation. The gloss controlled decorative wearlayer surface exhibits many performance properties sought in a flooring material.
Therefore, an object of the present invention is to provide a process of making a UV curable composition capable of producing high, medium and low gloss on surface covering products from the same composition, and a surface covering product resulting from this process.
Another object of the present invention is to provide a process to produce a variable gloss wearlayer surface on a floor covering product that does not require the usage of both long and short wavelength irradiation to control the gloss level.
A further object of the present invention is to provide a process of making a differential gloss product comprised of a discontinuous controlled gloss layer or a continuous controlled gloss layer.
Another object of the invention is to provide a flooring product including a high gloss and low gloss wearlayer surface.
Other objects, features, and advantages of the present invention will become apparent after review of the hereinafter set forth Brief Description of the Drawing, Detailed Description of the Invention, and the Claims.